The purpose of this project is to establish a new system for investigating the development of brain and behavior. There currently are well established avian and mammalian models of sexual differentiation. In each of these, investigators are studying the role of steroid hormones in the development of sex differences in brain and behavior. Some common themes exist, but there are also some remarkable differences among the systems. It is therefore critical to our understanding of steroid regulation of brain and behavior to determine which results can be generalized and which are specific to a particular animal model. My goal is to develop a reptilian model system comparable to the avian and mammalian systems that have been established. The data obtained from this system will increase the understanding of the evolution of the hormonal regulation of sex differences in brain and behavior and will add a comparative perspective to this general area of research. As in many vertebrates, the courtship behavior of the green anole (Anolis carolinensis) is sexually dimorphic. Males exhibit a series of behaviors directed to females, including the extension of a bright red throat fan, called a dewlap. This structure is extremely small in females. It is also not colored, and it is not normally displayed by females. The courtship behavior is regulated primarily by androgens in adulthood, but nothing is known about the sexual differentiation of the dewlap. In fact, little is known about the extent of the sexual dimorphism. The proposed experiments will investigate whether sex differences exist in the muscles and motoneurons that control dewlap extension. They then will begin to test the mechanism of steroid hormone action in the regulation of the behavior by determining whether there are androgen or estrogen receptors in those motoneurons, whether a dewlap can be induced in females by testosterone treatment during development and whether there are sex differences in steroid metabolizing enzymes in the brain, particularly in the general area of the motoneurons.